The invention relates to a radiation-sensitive semiconductor device having a semiconductor body comprising at least two radiation-sensitive diodes, on which a luminous spot can be imaged, and means for intensifying the photocurrent generated by the luminous spot.
The invention further relates to a focusing arrangement comprising such a photosensitive device.
A photosensitive semiconductor device of the kind mentioned is described in German Offenlegungsschrift DE-A 3706252.
Such devices are used in many fields and in different ways. More particularly they are used in the field of reading and writing optical information, such as in DOR ("Direct Optical Recording"), VLP ("Video Long-Play") and CD ("Compact Disk") apparatuses and for detecting excursions in radial and axial direction of the information-carrying medium, in this case the DOR, VLP or CD record.
In an application of the said type, the radiation-sensitive semiconductor device forms part, for example, of a Foucault focusing system, as described inter alia in "Philips Technical Review, Vol. 40, 1981/82, No. 9, pp. 266-272. In this case, the radiation beam of a laser focused on the CD or video record and reflected by the record is deflected by a semi-transparent mirror surface and is split up by wedge-shaped prisms into two beams, which are each imaged as a luminous spot on two photodiodes of a photodetector provided on a semiconductor wafer mutually separated by a narrow strip.
By means of a circuit, a tracking error signal is derived from the signal difference of the photodiodes, which signal is used as control signal for stabilizing the position of the focused laser beam with respect to the record. At the same time, a second tracking error signal can be derived from the signals generated by the photodiodes, which signal takes into account and ageing or dirtying of the optical system. In the said article in "Philips Technical Review", this is extensively explained so that reference may be made to this publication for further details.
Since the information on the record has the form of pits and non-pits having dimensions of 1 .mu.m or smaller, the device must be able to follow variations of high frequency in the detected luminous intensity. Moreover, the generated photocurrents must have a sufficiently high signal-to-noise ratio and must therefore not be too weak.
In general, photodiodes will be sufficiently rapid. The photocurrents supplied by the diodes are in most cases too weak, however, and must therefore be amplified with the aid of the aforementioned (switching) means.
The said means can be constituted in a simple manner by transistors. For this purpose, an emitter zone can be provided in the photodiodes so that the latter change into phototransistors, which supply an amplified signal, which has the desired signal-to-noise ratio.
A great disadvantage of this solution is that the transistors are then located at a comparatively large relative distance. As a result, because of the inevitable inhomogeneities of the semiconductor material it is difficult to make the transistor properties equal within the relevant tolerances.